Electrical condenser



Feb 9 1954 P. ROBINSON 2,668,936

ELECTRICAL coNDENsER f Filed May 26, 1948 PRES 70N Ro/No/v JNVENTOR.v

BYWQL Patented Feb. 9, 1954 STATES attesta O FFI CE ELECTRICAL CQNDENSER;

Preston Robinson, W`xl1iamstown-, Massi, assignor'" to Sprague Electric Company', North' Adams; Massi', a; corporation of Massachusetts applicati@Mayi 26, 1948,]seria11Ndaa3oi.

My. present. invention relates. totimproved electrical cond'ensers. More particularly, it `concerns stacked. and.. rolled. condensersy having a. high electrical capacity per unit of volume. Rolled paper. and. stacked. mica. condensers. can. be and. are.. produced. in large. quantities at low cost They are. satisfactory for.. most4 applications. in which a, condenser is required. However, for. some. applications. it ishighlyr desir-able or. even necessary tolprovidea condenser4 having greater, capacity per. unit of voluine,y than. can be. achieved, by these.. conventional types.

Rolled paper condensers generally employ electrode.. foils. o1` about. .00.0%'1 thickness. The insulation'. usually consists,n of' t'vvo or three layers of.' paper of perhaps. .0.00. thicknesseach. ln such condensersaswell?v asin. the stacked mica condensersamargin of atleast-.057, i. e.y excess. Widthy and length. ofA dielectric spacer,v is employed, in order to. reduce the possibility of breakdown between adjacentelectrode ioiledges. Such excess..ammintsl of dielectric` spacer mate.- rial,. of. course tendto limit. the. efficiency of. suoli condenser-s, i. e.,their. capacity perunit of volume. This, makes-it, impossible to. produce Verysmall unitsof' the, conventional types.I having adequate capacityv for mostpurposes.

Numerous. attempts. have been;v made to., pro,- duce. electrical condensers` of. high. capacity and small. volume. by dipping electrode foils in waxes andlacquersrto produce upon cooling; or. removal of. the. solvent,` anl insulated foil which. can` be Wound or stacked. with. other. foils to produce condensers4 Without.. they use of. the.4 customary, separate, dielectric. spacer sheets. These` attempts have been, unsuccessful,Y principally, because of the diiculty in providing uniform insulation abouttheedges andl at the cornersrof the.,coated foils.

The. electrode. foils employedin` highly-v efcient condensers must necessarily. be.- veryN thin; and have. sharp edges. When` coating.. insulation upon suchA ais-hin .-foil,y either by dipping; the lat.- ter in a solution, emulsion or suspensionof. the insulating. material, or by, electrophoreticA deposition4 of the insulating. material, itis .f oundthat the coating doeg notformuniiorrnly about. the edges and at the corners oftlefoil. In..most cases the insulation will pullH away" at. these locations, leaving an exposed or a poorly insulated Ioil surface. Unfortunately; itis at` the edges andvcornersof'the electrode foil", that' th'e eld strength is'a't the highest, so that'these'arethe locations Where breakdown isrnost likely-to ocour;` orwax'coated'electrode foils, the ed'geidiiculti'es have made it impractical; ifnutimpossible, to

In the prior' attempts toy employ; lacquer produce smal1,roll`ed' condensers with adequately insulatedfoilledges. If oneuinsulates large strips of' condenser` foil andA then punches electrodeV el'ejments thereiiom,.in order to produce astacked condenser, the difficulties.A of' obtaining. adequate insulation along the edges and at. the corners are even greater.V

lt has also been attempted' to insulate electrode foils with ceramic coatings. Suciently exible ceramic' coatings can beA produced on el'ctrode foilsf by electrophoretic deposition, combined with or. followed by a treatment" With' a resin or other suitable. binder'. However, the difficulties in securing 1 a'. unit vcrm anddurable coating). along the edges and at' the cornersof' the electrode toil are very great. Specially` designedjcathodes may be employed' during electrophoresis to crease the'v deposition' at the' edges. and corners ofthe foilbut such procedures are difficult to control properly.'y Oi course;A foils so insulated are subject' to the `saine. disadvantagesas the lacquered and".waxed foils;A when the electrode elements are punched,- or cut; from long` stri'ps in the' production of stacked l condensers.

It is: anlobje'ct of the present invention to over-- come the foregoing and related. disadvantages oft the` prior art condensers. A further objectis to' produce improved rolled andi stacked electrostatic' condensers. A'. stilll further' object' is to produce small' electrical" condensers' of relatively intgli capacity.. Another object is to provide ya simpletmethod for 'insulating the` edges and 'cor'- ners of. electrode elements" in' stacked" and' rolled electricalcondensers;

TheseA obiectsareattained' in accordance with the.' invention by" providingv an electrical con'F denser-'comprising' at? least two cooperating. electrodefil's, at' least one ofr` which is 'composedV4 of ai nlm-formingim'etal and'is insulated' onlits. nat surfacest'with' aiwaterresistant dielectric.. mate# rial and' on" its edges; With the electrolyt'i'call'y iorineidoxide ofsaid'metal. In amore restricted sensethisihvention i'sjconcerned'witli an' electri'cal condenser" comprising at' least two. thin, cooperating; electrode .'foil's,1 atl least' one ofwliich consists' of aluminum andAI is. insulatedv onA the flat surfaces thereof with a Water-resistant electro'lyticallv'" formed aluminum. oxide. In one of its' preerred' embodiments the invention' ist con'- cern'ed' With" an electrical' condenser comprising tvvo-thin c'onvolutelv` Wound', aluminum foils,y the ii'at" surfaces of at least" one' of'sai'dfioilsbeing insulated.' with. a flexible;` water-resistant dielectricmateri'al, andtheledges and corners of said foils'Y beingj insulated.: with" electrolvti'callv frrn'e'd aluminum oxide. This invention vis4 also con'cerned'witn amr-ccess'` for producing electrical condensers which comprises convolutely winding film-forming electrode foils, the nat surfaces of at least one of which are coated with a iiexible, Water-resistant, dielectric material, and thereafter forming an oxide film on the edges of said rolled foils in the presence of a film-forming electrolyte.

According to my invention I have found it possible to produce novel stacked and rolled condensers of the insulated foil type wherein the edges of the electrode foils are adequately insulated to provideA a uniform and relatively high voltage breakdown. More particularly, I have found that it is possible to form an oxide film on the foil edges by electrolytic means when the 4 the foil from a solution of the partially polymerized material.

The original dielectric coating preferably has a thickness of .0005" or less, in order that the maximum electrical capacity per unit of volume of the resulting condenser may be attained.

The coated foils are then processed, either before or after assembly into a condenser structure, to form the oxide film upon all exposed metal surfaces thereof, particularly at the edges and corners. This is accomplished by connecting the foil to a positive terminal of a Source of electric current and placing the so-connected foil foil is composed of a film-forming metal, despite the presence of insulation substantially covering the flat surfaces of the foil. By my process it is possible to produce electrostatic condensers having a high capacity per unit of volume without encountering the difficulties generally experienced with lacquered or otherwise insulated electrode foils. The temperature to which the foil need be subjected is relatively low, e. g., around 85 C.,

while the electrolyte used to form the oxide lm does not cause the original insulation any appreciable damage.

' Among the film-forming metals which may be used as the electrode material may be mentioned aluminum, tantalum, and titanium. I prefer to f use aluminum, since it is extremely exible and can be obtained in very thin sheets at relatively low cost. As a general rule the thickness of the electrode foil should be less than about .001 and preferably less than about .0005". It is with thicknesses of .0005 or less that my invention has its greatest utility since it is diiicult to form, that is, provide an in situ formed oxide on, aluminum of this thickness. According to my invention it is possible to oxidize all exposed metal surfaces at the edges and corners of the aluminum foils, after assembly of the condenser elements, without danger of cracking or flaking oif the formed oxide lm. While the oxide film is generally very thin relative to other types of insulation, it has a high breakdown strength when properly prepared, and it is thus possible to eliminate the relatively wide margins conventionally employed in the manufacture of stacked and rolled electrostatic condensers. is preferably of high purity, e. g. such as 99.85%.

The dielectric coating initially provided on the flat surfaces of the electrode foils may be organic or inorganic in nature, provided that it is resistant to the processing conditions met and the electrolyte used in the formation of the oxide lm on the edges of the foil. Since the condensers of the invention have particular advantage and utility at temperatures above about 125 C., high temperature resistant dielectrics are desirable, although not required in the practice of the invention. Among the suitable organic dielectric materials are the polytetrahaloethylenes and copolymers of tetrahaloethylenes with other polymerizable materials; polypentachlorostyrene and its copolymers; polyamides; polyurethanes; various high temperature condensation resins, and the like. These may be applied to the electrode foil by solvent lacquers, in emulsions, by spraying and by electrophoresis from suspensions of ceramic particles, inorganic paints, etc. Also suitable are the hydrolysis products of the aryl-,

The aluminum alkyland aralkylchlor-silanes, Which have become known technically as the silicones and polyin a nlm-forming electrolyte having a cathode therein. When processing an already assembled Condenser structure, both terminals of the unit may be connected to the positive energy source, so that all electrode foils serve as anodes in the resultant electrolytic cell.

The nlm-forming electrolyte consists of an ionogen, such as boric acid, oxalic acid and the like, dissolved in water or some other suitable liquid solvent.

For high voltage applications, I prefer to conduct the oxide formation in an ionogen solution with a pH greater than about 4.5. Representative ionogens for this purpose are boric acid and ycitric acid. The formation voltage may be on the order of 550 volts and the thickness of the film is determined by this voltage. The oxide film formed in this manner is dense, thin and adherent.

For low voltage applications, the oxide film may be formed in an ionogen solution possessing a pH less than about 4.5. Representative ionogens for this purpose are oxalic acid, sulfuric acid, succinic acid and chromic acid. The formation is usually Aconducted at about 10 to about 50 volts and low temperatures are satisfactory. The lm thus formed is porous and its thickness is dependent largely upon the total electrical energy supplied.

The foil to be edge insulated is electrically connected as an anode and placed in the electrolyte bath at the proper formation voltage. When the oxide film has been formed to the proper state, the foil is removed, washed and dried. A wound or stacked condenser body may likewise be treated, one or all of the foils being connected as anodes in the process. An advantage of this treatment is that it will serve to correct any pin holes, cracks or like iiaws that may be present in the original dielectric coating on the foil. Upon completion of the oxide lm formation-the fully insulated foil or condenser unit is preferably washed with distilled water and carefully driedI in an oven.

Reference is made for purposes of illustration to the appended drawing in which Figure-1 represents a cross-sectional View of a partially insulated, electrode foil,

Figure 2 represents a cross-sectional View of the foil of Figure 1, after the edge insulation has been formed,

Figure 3 represents a partial cross-section of a convolutely wound condenser assembly prior to insulation of the edges, and

Figure 4 represents a partial cross-section of condenser of Figure 3, after it has been processed in accordance with the invention.

Referring more specically to Figure l, Ill represents a metal electrode foil, the fiat surfaces of which are coated With insulation l2 and I 3. The latter, represented in Figure 1 as inorganic, may be applied by any of the known methods. as discussed previously.

Likewise, large strips of foil may be coated with dielectric and subsequently punched or cut to form small strips or squares suitable for stacking or rolling into small condensers. The disadvantage of these methods of insulation lies in the fact that the edge Il of conductor l0 is inadequately or not at all insulated. This may be attributed to the thinness of the foil (e. g., .0004") and the surface tension effect, when the dielectric coating is applied by conventional means. While the edge of insulation l2 and I3 is shown as corresponding to edge Il of foil IU, this is not necessarily the case, and, indeed, will probably occur only when the foil is punched or cut from larger strips.

Figure 2 shows the foil of Figure 1, after it has been provided with the edge insulation, in accordance with my invention. I4 and l5 represent the metal oxide film formed on the edge of the foil I0 by electrolytic action in a filmforming electrolyte.

Such an edge-insulated foil can be used to produce stacked or rolled condensers. If the operating voltage level is to be relatively high, the other and cooperating conductor may also be insulated in a similar manner. However, for many purposes, one insulated and one uninsulated foil may be used as electrodes, giving the maximum electrical capacity per unit of volume.

A typical stack-type construction might utilize aluminum foil of .0004" thickness, coated on each flat surface with insulation such as a ilexible, water-resistant, ceramic layer .00025 thick, and a cooperating electrode foil of aluminum of about .00025" thickness.

Figure 3 shows a rolled electrical condenser 20 in which 2l and 22 represent metal electrode foils, each coated on their flat surfaces with in sulation 24. The latter may be inorganic or, as here shown, a thin layer of resinous material, such as a polytetrahaloethylene, a silicone resin or a high melting polyisocyanate or polyamide. The condenser section is wound, so that the two electrode foils 2| and 22 overlap completely, thus giving maximum capacity per width of foil. The edges, however, such as edge 23 of foil 2|, are exposed for reasons heretofore mentioned. A terminal tab 25 is attached to foil 22 and a tab 27 attached to foil 2l. These tabs may be of aluminum or other film forming metal.

Figure 4 shows the rolled condenser of Figure 3, after it has been subjected to the electrolytic treatment described above. The exposed edges of the electrode foils are now insulated by the lm of metal oxide so formed, as is indicated by 26 on edge 23. If tabs 25 and 2l are of aluminum, they also will be provided with an oxide film. For this reason, it is possible to coat a portion of each with a wax or resin prior to the formation process. Thereafter the wax or resin may be removed.

The edge insulation of the invention is stable over the normal temperature range. It resists deterioration due to corrosive atmospheres and is flame-proof, being wholly inorganic in nature. These properties, in combination with the adherence obtained by following the instructions given herein, make the insulation and condensers of the invention useful over a wide range of temperatures and conditions without sacrificing the high volume eiciency achieved. The advantage in being able to treat a rolled or stacked condenser after assembly is of great value, since cracks or defects caused during assembly may be corrected.

As many widely different embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments hereof except as defined in the appended claims.

I claim:

1. An electrostatic condenser of high volumetric efnciency comprising electrode foils of opposite polarity, each electrode having a thickness of less than 0.0005 inch, said foil surfaces being metal oxide-free and sandwiched between layers less than 0.005 inch thick of dielectric material, and said foils being superimposed substantially exactly over each other so that their edges are adjacent, at least one of the foils being of filmforming metal and having on its adjacent edges an in situ electrolytically-formed oxide of said nlm-forming metal.

2. An electrostatic condenser as defined by claim 1 wherein the two electrode foils are convolutely wound in insulated relationship to each other with the dielectric layer, and the filmforming metal is aluminum.

3. An electrostatic condenser as defined by claim 1 in which the dielectric material is a polytetrahaloethylene.

4. An electrostatic condenser as defined by claim 1 in which the dielectric material is a ceramic.

5. The combination as defined by claim 1 in which both electrode foils are of film-forming metal.

6. The combination as defined by claim 1 in which both electrode foils are of film-forming metaly and both have their edges coated with in situ electrolytically formed oxide.

7. The combination as defined by claim 1 in which the dielectric layer is in the form of a coating on one foil.

8. An electrostatic-rolled condenser having two elongated foils of opposite polarity each less than 0.0905 inch thick convolutely wound in insulated relationship to each other between layers of metal oxide-free dielectric material having a thickness of 0.0005 inch, said foils being substantially exactly aligned so that their long edges, at which during use the electric field between the foils tends to concentrate, are adjacent each other, at least one foil being of film-forming metal and having its long edges coated with an in situ electrolytically-formed layer of oxide.

PRESTON ROBINSON.

References Cited in the file 0f this patent UNITED STATES PATENTS Number Name Date 1,846,844 Clark Feb. 23, 1932 2,214,876 Clark Sept. 17, 1940 2,222,195 Elsey Nov. 19, 1940 2,238,031 Brennan Apr. 15, 1941 2,296,616 Koller Sept. 22, 1942 2,374,449 Mulcahy Apr. 24, 1945 2,393,966 Brennan Feb. 9, 1946 2,408,790 Mack Oct. 8, 1946 2,408,910 Burnham Oct. 8, 1946 2,421,652 Robinson June 3, 1947 2,447,388 Antonol Aug. 17, 1948 FOREIGN PATENTS Number Country Date 474,063 Great Britain Jan. 17, 1936 584,851 Great Britain Jan. 24, 1947 

1. AN ELECTROSTATIC CONDENSER OF HIGH VOLUMETRIC EFFICIENCY COMPRISING ELECTRODE FOILS OF OPPOSITE POLARITY EACH ELECTRODE HAVING A THICKNESS OF LESS THAN 0.0005 INCH, SAID FOIL SURFACES BEING METAL OXIDE-FREE AND SANDWICHED BETWEEN LAYERS LESS THAN 0.0005 INCH THICK OF DIELECTRIC MATERIAL, AND SAID FOILS BEING SUPERIMPOSED SUBSTANTIALLY EXACTLY OVER EACH OTHER SO THAT THEIR EDGES ARE ADJACENT, AT LEAST ONE OF THE FOILS BEING OF FILMFORMING METAL AND HAVING ON ITS ADJACENT EDGES AN IN SITU ELECTROLYTICALLY-FORMED OXIDE OF SAID FILM-FORMING METAL. 